1. Field of the Invention
This invention relates to an incremental encoder for use in a highly accurate length or angle measuring apparatus for industry such as FA, and particularly to an encoder for applying light to an object and utilizing information of the light obtained from the object.
2. Related Background Art
In a light pickup or an optical measuring apparatus in a highly accurate positioning apparatus for industry such as FA, there has heretofore been widely utilized an encoder for irradiating an object, receiving transmitted light through or reflected light from the object, and obtaining information of the object which transmits or reflects light. In the encoder, any positional deviation is converted into a pulse train, and this pulse number is counted to thereby detect the relative positional deviation, but the absolute position is found by generating an origin position signal, and resetting the counter to zero by the origin signal.
The above-described encoder is called an incremental encoder, and a linear encoder for detecting linear displacement and a rotary encoder for detecting rotational displacement are known. What are required of these optical measuring apparatuses today are items such as downsizing and higher accuracy.
In recent years, in the field of FA, in compliance with the requirement for higher accuracy, there has been adopted an encoder of the xe2x80x9cgrating interference typexe2x80x9d which detects the generation of an incremental encoder signal by the utilization of the phase shift of the wave front of diffracted light by the relative movement of a diffraction grating. In the encoder of the xe2x80x9cgrating interference typexe2x80x9d, a grating of a minute pitch of the order of xcexcm is utilized, and a value of sub xcexcm is achieved as resolving power per pulse (period).
In the field of the above-described detecting apparatus utilizing light, and particularly the field of displacement detection, the accuracy of the measurement of the origin which is the reference position has come to be required as higher accuracy is advanced. In the origin detection of an incremental encoder of high resolving power, in order to secure the same degree of resolving power, there has been adopted a method of detecting the origin by applying to a minute slit pattern formed on a scale or a disc at the order of xcexcm a minute condensed light beam of the same degree of size as the minute slit.
Referring to FIG. 1 of the accompanying drawings which shows the reference position measuring technique of an optical type rotary encoder disclosed in Japanese Patent Application Laid-Open No. 2-93324, a light beam emitted from a light-emitting element 1 passes through a collimator lens 2, and is reflected by a half mirror 3, and a light beam transmitted through the half mirror 3 is reflected by a mirror 4, and the two light beams are reflected by half mirrors 5a and 5b, respectively, are linearly condensed by cylindrical lenses 6a and 6b, and are incident on a disc 7. When the disc 7 is rotated and slits 8a and 8b formed on the disc 7 are irradiated by the light beams, the light beams reflected by the slits 8a and 8b are incident on light receiving elements 9a and 9b, and an origin signal is outputted.
Also, Japanese Patent Application Laid-Open No. 3-115920 discloses an encoder in which slits are disposed while being deviated from each other.
In the embodiment of the origin signal of an optical type linear encoder disclosed in Japanese Patent Application Laid-Open No. 7-294214, a light beam emitted from a light-emitting element is incident on the grating portion of a scale, whereupon it is reflected and diffracted, and the light beam is incident on a light receiving element. When the scale is moved and the end of a slit goes beyond a portion being irradiated by the light beam, the light beam is no longer incident on a sensor. By the above-described principle, an origin signal is outputted.
In the origin measuring sensor of the encoder according to the prior art, when the encoder itself is made considerably compact and higher in accuracy, for example, the adjustment for making a minute condensed light beam for detection coincident with a minute slit pattern in the detection of the origin signal is delicate, and high accuracy is required of the mutual position adjustment of the scale and the measuring sensor.
In some of highly accurate grating interference methods, in order to make detection accuracy good, a minute slit has been detected at different timings by two light receiving elements positionally deviated relative to each other, and an origin signal has been generated by the utilization of the two differential signals. In this case, when a method of generating a pulse waveform as the origin signal when the levels of the two signals coincide with each other is adopted, the origin is defined as xe2x80x9cthe edge portion of the rising (falling) of the pulsexe2x80x9d.
On the other hand, in an incremental encoder of low accuracy, usually the detection signal of a slit is utilized as an origin signal synchronized with the waveform of one of incremental signals of A and B phases.
Both in the highly accurate grating interference methods and in the incremental encoder of low accuracy, there is required the production of a signal providing the information of the origin position which is better in accuracy and stable and easy to decide upon the pulse width. Particularly when the incremental signal is minute as the signal providing the information of the origin, there is required a signal stably having a width equal to that of this incremental signal. Further, even when the output positions of the incremental signal and the origin information signal are deviated from each other as by a variation in the quantity of light of a light source portion, it is required that synchronism can be taken stably.
A first object of the present invention is to provide an encoder of a construction in which a signal of a pulse width equal to that of an incremental signal can be stably provided as a signal providing the information of the origin.
A second object of the present invention is to provide an encoder in which, in addition to the foregoing, synchronism between the incremental signal and the signal having the information of the origin can be stably taken irrespective of any fluctuation in the quantity of light or the like.
Another object of the present invention is to provide a rotary encoder in which a stable origin signal accurately synchronizable with a minute displacement signal irrespective of the vibration or eccentricity of a disc is produced and synchronized with a rotational displacement signal.
Further objects of the present invention will become apparent from the following description of the embodiments of the invention.